Natural gas is widely used in industrial and residential applications. Transportation of gas is more difficult than liquid transportation. When natural gas fields are relatively close to the users, the gas is usually transported by pipeline. In other cases where gas fields are remotely located and/or the users are far away from the fields, the natural gas is first liquefied, and then transported in the form of liquefied natural gas (LNG).
In a LNG plant, contaminants, such as carbon dioxide and hydrogen sulfide, have to be reduced to very low levels. For example, the carbon dioxide content in the feed gas stream must be less than 50 ppmv before liquefaction to avoid formation of dry ice within the system. Commercially, this can be achieved by using a solvent absorption process, such as contacting the natural gas with an amine solvent, such as monoethanol amine (MEA) or diethanol amine (DEA) for example, to remove the carbon dioxide. The amine is regenerated after use. Other CO2 removal processes are known in the art, such as cryogenic processes, adsorption processes such as pressure swing adsorption (PSA) and thermal swing adsorption (TSA), and membrane-based processes.
The contaminant removal is followed by the natural gas being sent through a molecular sieve dehydration unit to remove water to below 1 ppmv.
In recent years, there has been increasing interest in developing floating LNG (FLNG) facilities that can liquefy the offshore gas for transportation. A floating LNG facility can be positioned adjacent to an offshore natural gas well to liquefy the gas as it is being loaded on a tanker, which eliminates the need for pipelines to take the gas onshore prior to liquefaction in a conventional facility. The floating LNG facility could be moved from one port to another to service small LNG fields, as needed.
However, the processing of natural gas on such a vessel entails problems not encountered by land-based facilities. The movement of the ship can result in poor distribution of liquid in a separation column. The mass transfer efficiency of the gas-liquid phase inside the column will be significantly reduced due to the poor distribution. In FLNG pretreatment, an amine absorber and regenerator are used to remove acid gas. If the column efficiency is reduced, the treated gas from the amine absorber may not be able to meet the low acid gas specification (e.g., less than 50 ppm CO2), which will generate a plugging problem of the downstream liquefaction system.
Therefore, there is a need for a reliable process that can ensure that the treated gas meets the LNG feed specification.